Lensed Fabry-Perot interferometers (FPIs) have provided seminal optical spectrum measurements in physics, chemistry, astronomy and other diverse scientific fields for more than a century. Miniature lensed FPIs adapted to fiber optical systems with optical fiber pigtails can provide medium resolution tuning (finesse=100). Lensless fiber Fabry-Perot interferometers (FFPIs), however, can perform at resolutions&gt;500 for tuning functions in optical fiber systems. These high performance tunable FFPIs (also called fiber Fabry-Perot tunable filters(FFP-TFs) have made interrogator systems for accurately measuring wavelength response of passive or active fiber optics devices possible.
In wavelength sensing applications over a wide wavelength range, the high resolution of tunable FFPIs provides a distinct advantage and allows sensing to picometer levels. In addition, the all-fiber, lensless FFPI is mechanically robust with a reliability record in the field of less than 300 FITS (failure interval times) and, when appropriately temperature compensated, operates from -20.degree. C. to +80.degree. C. with minimal insertion loss and tuning voltage variation.
All-fiber lensless FFPIs used as scanning interferometers can sense extremely small wavelength shifts when piezoelectric actuators (PZTs) are used for tuning the multipass dual mirror optical cavity. However, early work using these interferometers for measuring wavelength shifts in reflected FBGs lacked adequate wavelength references for long-term stability. See: A. D. Kersey et al. (1993) supra; E. J. Friebele et al. (1994) supra; A. D. Kersey et al. (1995) supra; A. D. Kersey (1996) supra.
The PZT piezoelectric actuator typically employed to tune FPIs exhibits dynamic nonlinearities arising from nonlinear length dependence on voltage, and voltage hysteresis with temperature. Therefore, to fully utilize a tunable FPI (whether a lensed or all-fiber FPI) as a spectrally calibrated interferometer, certain real-time multi-wavelength referencing techniques should be implemented. Among various absolute wavelength references, molecular absorption lines of .sup.12 C.sub.2 H.sub.2 and .sup.13 C.sub.2 H.sub.2 acetylene (Sakai, T. et al. (1992), "Frequency stabilization of laser diodes using 1.51-1.55 .mu.m absorption lines of .sup.12 C.sub.2 H.sub.2 and .sup.13 C.sub.2 H.sub.2 " IEEE J. Quantum Electron. 28:75-81) seem to offer potential media for multi-wavelength referencing. However, their unevenly spaced absorption lines and absorptive operation render them difficult for tunable FPI-based spectral applications. FPIs with narrowly-spaced uniform transmission peaks (e.g. .about.100 GHz) have often been considered for absolute frequency reference when locked to atomic or molecular absorption lines (Glance, B. S. et al. (1988), "Densely spaced FDM coherent star network with optical signals confined to equally spaced frequencies," IEEE J. Lightwave Technol. LT-6:1770-1781; Boucher, R. et al. (1992), "Calibrated Fabry-Perot etalon as an absolute frequency reference for OFDM communications," IEEE Photonics Technol. Lett. 4:801-803; Gamache, C. et al. (1996), "An optical frequency scale in exact multiples of 100 GHz for standardization of multifrequency communications," IEEE Photonics Technol. Lett. 8:290-292). While this is appropriate for stabilizing laser arrays, it is restricted in wavelength positioning and requires active feedback control. Sampled fiber Bragg gratings (FBGs) have recently been reported to exhibit uniform optical frequency scale of 100 GHz spacing useful for WDM applications (Martin, J. et al. (1997), "Use of a sampled Bragg grating as an in-fiber optical resonator for the realization of a referencing optical frequency scale for WDM communications," Optical Fiber Communication Conf. OFC-97, Technical Digest, paper ThI5, pp. 284-285). However, their narrow overall wavelength span poses a limitation.
In-Fiber Bragg gratings (FBGs) can produce a narrow-band response around a single wavelength (reflecting a narrow-band peak or transmitting the illuminating spectrum with a narrow-band notch or bole). The dopants used to increase the index of refraction in the cores of optical fibers are photosensitive and by exposing a single-mode fiber to interfering beams of UV light, or through a suitable mask, a diffraction pattern can be written into the core that reflects a single narrow-band wavelength of light. The resulting fiber Bragg grating (FBG) transmits all other wavelengths carried by the single-mode fiber and reflects almost all (up to 99.9%) of the light that meets the Bragg condition (i.e., the Bragg reflection wavelength .lambda..sub.B =2 ns, where s is the grating pitch and n is the effective index of the fiber core).
FBGs have been employed in fiber optic sensors in a variety of sensing application, including for strain and temperature measurements. See, for example, U.S. Pat. No. 4,996,419 (Morey) issued Feb. 26, 1991; U.S. Pat. No. 5,380,995 (Udd et al.) issued Jan. 10, 1995; U.S. Pat. No. 5,397,891 (Udd et al.) issued Mar. 14, 1995; U.S. Pat. No. 5,401,956 (Dunphy and Falkowich) issued Mar. 28, 1995; U.S. Pat. No. 5,410,404 (Kersey et al. ) issued Apr. 25, 1995; U.S. Pat. No. 5,426,297 (Dunphy and Falkowich) issued Jun. 20, 1995; U.S. Pat. No. 5,591,965 (Udd) issued Jan. 7, 1997. See also, for example, FBG sensor applications in: (embedded FBGs) P. D. Foote (1994) "Fibre Bragg Grating Strain Sensors for Aerospace Smart Structures" Presented at the Second European Conf. on Smart Structures and Materials, Glasgow, U. K., session 8, p. 290 and references therein. Multiparameter environmental sensors can be formed using dual overwritten FBGs as described in U.S. Pat. No. 5,591,965. FBGs can be employed to form in-fiber lasers which can be used for sensing applications as described in U.S. Pat. No. 5,513,913 (Ball et al.) issued May 7, 1996. These sensing techniques depend on the ability to accurately measure the wavelengths of light reflected or transmitted by FBG sensors. A number of sensing system configurations have been developed. See, for example, U.S. Pat. No. 5,361,30 (Kersey et al.) issued Nov. 1, 1994 and U.S. Pat. No. 5,410,404 (Kersey et al.) issued Apr. 25, 1995; A. D. Kersey et al. (1993) "Multiplexed fiber Bragg grating strain-sensor system with a fiber Fabry-Perot wavelength filter." Optics Letters 18(16):1370; E. J. Friebele et al. (1994) "Fiberoptic Sensors measure up for smart structures" Laser Focus World (May) pp. 165-169; A. D. Kersey et al. (1995) "Development of Fiber Sensors for Structural Monitoring" SPIE 2456:262-268 (0-8194-1809-9/95); A. D. Kersey (1996) "Interrogation and Multiplexing Techniques for Fiber Bragg Grating Strain-Sensors" Optical Sciences Division Naval Research Laboratory (NRL) code 5674 distributed by NRL at SPIE Meeting Fall 1996 (Denver, Colo.); U.S. Pat. No. 5,426,297 (J. R. Dunphy et al.) issued Jun. 20, 1995; Y-J. Rao and D. A. Jackson (1996) "Universal Fiber-Optic Point Sensor System for Quasi-Static Absolute Measurements of Multiparameters Exploiting Low Coherence Interrogation" J. Lightwave Technol. 14(4):592-600; Y-J. Rao et al. (1996) "Strain sensing of modern composite materials with a spatial/wavelength-division multiplexed fiber grating network" Optics Letters 21(9):683-685; and references cited therein. FFPIs and other fiber-based interferometers can also be used as sensing elements in environmental sensor, for example for temperature and strain sensing. Typically, an environmental (e.g.,temperature or strain) change induces a change in the length of the filter cavity which is exhibited as a shift in filter wavelengths.
Low resolution (finesse) fixed FPIs (lensed or all-fiber lensless) can be used as accurate wavelength references for the calibration of tunable FFPIs (e.g., FFP-TFs) to increase both accuracy, and long-term stability of wavelength measurements. Although FPIs produce multiple, very accurately spaced, wavelengths (i.e., a comb of peaks), a consistent problem with their use as wavelength references has been the need to accurately identify a given individual peak among the multiple wavelength peaks produced. Hence, in the application of fixed-cavity FPIs as wavelength references, a means for identifying the particular teeth in comb of the FPI output peaks is needed.
This invention provides a wavelength reference system which overcomes the difficulties encountered in the use of such interferometers, particularly in the use of fixed-cavity FPIs, as references in wavelength detection and measuring devices.
Fixed and tunable FFPIs are described in U.S. Pat. Nos. 5,062,684, 5,073,004, 5,212,745; 5,212,746; 5,289,552; 5,375,181; 5,422,970; 5,509,093 and 5,563,973, all of which are incorporated by reference in their entireties herein, particularly for their disclosure of the structures and operation of these filters.